Internal combustion engines fluidly couple to exhaust aftertreatment systems that purify exhaust gases generated as byproducts of combustion. Exhaust aftertreatment systems may include oxidation catalysts, reduction catalysts, selective catalytic reduction catalysts and particulate filters. Byproducts of combustion may include unburned hydrocarbons, carbon monoxide, nitrides of oxide, which may be referred to as NOx molecules, and particulate matter. Operation may be monitored by one or more sensing devices that are disposed in the exhaust gas feedstream, including, e.g., a NOx sensor. Operation may also be determined employing simulation models that dynamically execute during operation.
Selective catalytic reduction catalysts (SCRs) may employ reductants for reducing NOx molecules to elemental nitrogen. One known reductant is urea, which may be transformed in an exhaust system into ammonia (NH3). The reductant may be injected into the exhaust gas feedstream upstream of one or multiple selective catalytic reduction catalysts, and may be stored on a surface or otherwise captured for use in reducing NOx molecules to elemental nitrogen and water.
Signal output from a sensor that is configured to monitor particulate matter may exhibit a sensitivity to NH3 molecules when disposed to monitor an exhaust gas feedstream. A diagnostic monitoring system for evaluating efficiency of a particulate filter element may exhibit either a false-negative result or a false-positive result under certain operating conditions due to occurrence of NH3 in the exhaust gas feedstream.